# Working on a movie and have a question on observer and double slit experiment

Is it safe to conclude from the double slit experiment that the observer actually affects what is being observed? I ask with regards to a project I'm writing which I wish to be scientifically accurate.

Fredrik
Staff Emeritus
Gold Member
It is, but you you should probably explain your idea so we can tell you why it's wrong.

Until then, I can only offer some general comments: An observation is an interaction between the particle and its environment that creates a record of what just happened. So we shouldn't be surprised that it changes the state of the particle in some way. Explanations of the double-slit experiment for people who aren't physics students (like the famous Dr. Quantum video) can give you the wrong idea about that. They often suggest that observation is a passive act of "just looking", but it's certainly not.

What we can say may depend on some specific aspects of the experiment considered.
If you consider just one closed physical system then we can say yes, physically, the observation of its state requires to physically interact with it, and thus affect it.
For example if you have one electron and want to measure its spin in the up/down direction, you must interact with it so that if its spin was previously rightwards (which means : a 100% probability of finding it so if you measure it in the left-right direction), then the required interaction for the up/down measurement process has the physical effect of destroying the left/right component of the spin, and thus leading to equal chances for it to be found leftwards or rightwards if you measure it in the left-right direction after this.

This is also the case in the double-slit experiment : the observation of which slit a particle goes through, requires some sort of physical interaction with this particle when it goes through.
Such an explanation may turn out to be unsatisfactory when we consider some experiments where the effect on the probability of final result is greater than what seems to be the "probability of physically affecting the system", for example in the case when one slit is bigger than the other and you detect the particle going through the small slit.

Other cases may be ambiguous : in the EPR experiment, or in the Schrodinger's cat experiment when an observer would receive at distance the information on what happened to the cat (in the theoretical view that the cat did not already observe itself), observing one particle somehow "affects" the other at distance, but the relevance of the words is not clear, whether or not we can really say that measuring the one "affects" the other.
For this, it may be necessary to enter the effective mathematical expression of what quantum physics says, how it describes measurements, and how ambiguous it is on the interpretation. See my introduction to quantum physics (settheory.net/quantum-measurement ) for detailed explanations of how measurements are processed and how they "affect" systems.

Is it safe to conclude from the double slit experiment that the observer actually affects what is being observed? I ask with regards to a project I'm writing which I wish to be scientifically accurate.

the observer via the measurement (collapses the wave function and hence) effects what is being observed

you could have no human/living observer but a measurement and still (collapse the wave function and hence) effect what is being measured

not all agree with the above though.

the answer to your question depends upon what context you describe the experiment/movie in